Mercury (Hg) is a global health concern due to its toxicity and ubiquitous presence in the environment. Here we review current methods for measuring the forms of Hg in the atmosphere and models used to interpret these data. There are three operationally defined forms of atmospheric Hg: gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate bound mercury (PBM). There is relative confidence in GEM measurements (collection on a gold surface), but GOM (collection on potassium chloride (KCl)-coated denuder) and PBM (collected using various methods) are less well understood. Field and laboratory investigations suggest the methods to measure GOM and PBM are impacted by analytical interferences that vary with environmental setting (e.g., ozone, relative humidity), and GOM concentrations measured by the KCl-coated denuder can be too low by a factor of 1.6 to 12 depending on the chemical composition of GOM. The composition of GOM (e.g., HgBr2, HgCl2, HgBrOH) varies across space and time. This has important implications for refining existing measurement methods and developing new ones, model/measurement comparisons, model development, and assessing trends. Unclear features of previously published data may now be re-examined and possibly explained, which is demonstrated through a case study. Priorities for future research include identification of GOM compounds in ambient air and development of information on their chemical and physical properties and GOM and PBM calibration systems. With this information, identification of redox mechanisms and associated rate coefficients may be developed.